The WTA Championships bring together the top eight players in the world and begin with round-robin play Tuesday in Istanbul.

The top two finishers from each group advance to the semifinals.

Third-ranked Maria Sharapova has pulled out because of a shoulder injury and Wimbledon champion Marion Bartoli has retired.

Here are the five things to know about the tournament, which moves to Singapore for the next five years:

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THE GROUPS

The eight players are divided into two groups. The Red Group consists of Serena Williams, Petra Kvitova, Agnieszka Radwanska and Angelique Kerber. The White Group has Victoria Azarenka, Li Na, Sara Errani and Jelena Jankovic.

The top-ranked Williams is the heavy favorite going into the last big event of the season. The American is the defending champion and is coming off her best season with 10 titles.

Williams, making her eighth appearance in the event, is looking to become the first player to successfully defend the title since Justine Henin in 2007.

Williams has clinched the year-end No. 1 ranking for the third time in her career after 2002 and 2009. She won the French Open and U.S. Open this season and has 17 Grand Slam titles. She regained the No. 1 ranking in February.

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THE STREAK

The longest championships winning streak was compiled by Martina Navratilova, who won 21 straight matches in winning five titles between 1983 and 1986 and reaching the quarterfinals in 1987.

The second-longest win streaks in tournament history went to Chris Evert and Monica Seles, who each won 12 matches in a row.

Williams enters this year's tournament with a 10-match winning streak at the event, winning titles in 2009 and 2012 and not competing in 2010 and 2011. She also won the event in 2001.

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AZARENKA'S THREAT

If anyone can challenge Williams for the title, it should be the second-ranked Azarenka.

Azarenka, who won the Australian Open, is the only other 2013 Grand Slam champion in the event.

Williams has lost twice to Azarenka this year: in Doha in February, the day after regaining the top ranking from her rival, and in Cincinnati in August after leading for most of the match.

But Williams then defeated Azarenka to win the U.S. Open in the longest final in 30 years.

Azarenka, however, has been struggling recently, losing in the second round in Tokyo and in the first round in Beijing, when she had 15 double-faults.

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JANKOVIC'S COMEBACK

Jankovic, once ranked No. 1, is back in the top 10 for the first time since 2010 and will be playing her fifth WTA Championship.

She credits her brother Marko for helping her get there.

Jankovic said that at 28, she did not feel old.

"People like Serena, at 32, she's playing her best tennis and having the best year of her career, and I think that inspires all of us," Jankovic said.